Gastrointestinal (GI) motility results from organized contractions of the muscles that line the GI tract. The contractile patterns of GI motility result from omnipresent rhythmic electrical activity and modulation of this activity by neural and hormonal influences. Although the electrical activity of the GI tract has been termed "myogenic" because it persists in the absence of neural and hormonal inputs, many investigators now believe that pacemaker activity actually originates in a special class of cells known as interstitial cells of Cajal (ICs). In addition to serving as electrical pacemakers, ICs may participate in neuromuscular transmission because these cells are often closely associated with varicose nerve fibers. This project will combine several morphological and physiological techniques to examine the role of ICs in colonic electrical activity and neurotransmission. We will establish a better means of classifying the different types of ICs in GI muscles, by characterizing the expression of various structural and function proteins. We will also study the structure of IC networks and the innervation of ICs by intrinsic nerves. To determine the role of ICs in pacemaker activity, the voltage-dependent ion channels of ICs that may be important in pacemaker activity will be characterized and compared to ionic conductances in smooth muscle cells. A mathematical model describing electrical slow waves will be expanded to include the influence of pacemaker activity initiated by ICs. The structure of IC networks will be characterized during the development of electrical rhythmicity following birth and during the decay in electrical rhythmicity caused by agents that are toxic to ICs. The hypothesis that ICs are innervated will be explored histochemically and by studying the responses of these cells to neurotransmitter substances. The second messengers linking stimulation by neurotransmitters to cellular responses will also be studied. Recent evidence has suggested that ICs play a particularly important role in enteric inhibitory responses. ICs may amplify inhibitory neurotransmission by synthesis of nitric oxide (NO). NO and other transmitters may active NO synthesis in ICs. This pathway will be explored by studying cellular responses of freshly isolated and cultured ICs. NO production will be measured in response to several stimuli. This project will provide novel and important information about the physiology and morphology of ICs. Since these cells may be extremely important in the generation of electrical rhythmicity and neurotransmission, the experiments proposed are essential to a full understanding of GI motility.